Laser beams, without added optical devices, are received at a target as a single small spot. However, there are numerous applications where it is desirable to deliver the laser beam expanded linearly into a single line or two-dimensionally into a beam having larger dimensions. To do so, the laser beam is usually caused to pass through a conventional optical device such as lens, prism or combination of both. However, these optical devices have very limited expansion ratios (i.e. expanded beam size/original beam size) and the beam expanded thereby is unable to illuminate a target having very large dimensions.
It has been suggested to use fiber gratings to modify the nature of the laser beam reaching a desired target. A fiber grating is a flat array of transparent parallel, cylindrical fibers arranged side by side without any spacing therebetween (FIG. 1). Typical fibers, selected based on their known transmission ability for the selected wavelength of the laser beam being handled, include various glass fibers as well as plastic fibers such as polycarbonate or acrylic fibers, with or without added dopants. When a laser beam is incident on a number of fibers making up the fiber array perpendicularly to the axes of fibers, the fibers, each acting as a cylindrical lens, focus the incident beam into a series of spots, that are referred to as secondary laser beams (i.e. the Huygens principle). In other words, the fiber grating splits the incident beam into a number of secondary beams. These secondary beams interfere with one another and produce, when projected on a screen, a series of discrete diffraction dots separated by blank spaces that are distributed along an extremely wide arc. As the dispersion angles of these dots and the blank spaces measured from the center of the arc (i.e. from the optical axis of the incident laser beam) are constant regardless of the radius of the arc, the further the target area is away from the fiber grating, (that is, if the target is located in the so-called far field), the larger the dimensions of the dots as well as the blank spaces between two neighboring dots. Conversely, the nearer the target, the narrower the space between two neighboring dots and the smaller the size of each dot. If the target is located closer to the fiber grating (that is, if the target area is located in the so-called near field), the blank spaces between two dots disappear as the two diffraction dots come closer and eventually overlap at a certain radial distance from the center of the grating, making the resolution of two neighboring dots extremely difficult. For this reason, a train of diffraction dots turns into a continuous diffraction line at a critical radial distance in the near field. However, the fiber gratings of prior designs, which consist of the glass fibers of a same diameter, have a major deficiency. As shown in FIG. 2, such an array produces a series of bright, discrete, and equally spaced dots 12 separated by blank spaces 14 where much fainter images of ghosts 16 may appear. These ghosts brighten the blank space between two neighboring dots and impair the discreteness of each diffraction dot. If the fiber grating is designed to produce a train of discrete diffraction dots, the presence of ghosts between two diffraction dots is undesirable.
A primary objective of the invention is to make use of the ghosts. In particular, the invention is designed to intentionally change the systematic way the fibers of a same diameter are arranged to form a grating by laying the fibers of different diameters alternately. A fiber grating formed from the fibers of different diameters alters the spacing between the sources of secondary beams and makes the secondary beams interfere with one another in a more complex way than the case of the grating consisting of the fibers of a same diameter, thus creating more ghosts in the blank spaces and brightening up the spaces to turn a series of diffraction dots into a continuous arc of monochromatic light. The invention also includes a new technique to convert the Gaussian profile of an incident laser beam into the one with a substantially rectangular profile.
While the description of invention presented above is based on the incident laser beam being a single-mode laser beam (i.e. the laser beam having only one wavelength), such as a HeNe laser beam, the incident laser beam may also be a multimode laser beam (i.e. a beam consisting of multiple beams having nearly same, but different wavelengths), such as produced from some of the diode lasers. Multiple numbers of secondary beams having different wavelengths are produced by the fiber grating, causing them to interfere with one another in more complex ways than the fiber grating made of the same diameter fibers. As a result, a greater number of diffraction dots are produced than the case where the incident beam is from a single laser source and they fill up blank spaces 14 between discrete dots 12 with newly produced dots to turn a train of diffraction dots into a continuous line. This effect of multimode laser on the conversion of a dot train into a continuous diffraction line is evident in the near field, that is, when the target area is located less than 1 m (about 40 inches) or so from the grating. Because, the nearer the target area is located from the grating, the narrower the spacing between neighboring dots and the blank space diminishes and two diffraction dots overlap when the distance between the grating and the target becomes less than the critical distance as already mentioned above. For this reason, the combination of the fiber grating consisting of the fibers of a same diameter and multimode laser is useful in some near field applications such as the bar code reading system.
U.S. Pat. No. 5,113, 286 describes a typical prior use of a fiber gratings and diffraction gratings to expand a laser beam. The '286 patent describes the use of a diffraction grating apparatus to generate an array of an even number of spots when illuminated with a monochromatic plane wave of light. The fiber gratings devices disclosed prior to the invention in the '286 patent all generated an odd number of spots.
U.S. Pat. No. 5,345,336 is directed to an imaging device having a plurality of microspherical lenses. FIGS. 23, 24a and 24b which shows a prior used fiber grating comprising a plurality of parallel fibers in a plane overlapping a second set of parallel fibers with the fibers in the first set of fibers being at an angle to the second set. This arrangement, described as background, creates a series of blurred spots. The objective of the '336 invention is to produce a series of discrete sharply focused spots when illuminated by a single laser beam, an objective not obtainable with fiber gratings which produced discrete but indistinct spots.
U.S. Pat. No. 5,627,927 and U.S. Pat. No. 5,646,401 describe the use of one or more gratings formed from several parallel fibers as a sensor for environment conditions such as temperature or humidity. While the fiber gratings are not described in detail, from the description of the prior art therein it appears that the fibers in a particular grating are of uniform diameter but each different gratings may be composed of fibers of a different diameter. This patent utilizes a change in birefringence in the fiber which occurs in response to changes in ambient conditions or stress in the fibers. The fibers are illuminated by a relatively broadband or scattered light source and the spectral reflectivity and transmission of the gratings are observed, the changes therein being indicative of changing ambient conditions.
Photonics are used for civilian and military applications in several different ways. As an example of military applications, a laser beam emitted from a source shined on an object can be used to mark that object so that projectiles can be guided to that object or personnel can locate the object, such as a downed aircraft or missing water vessel, to rescue it. Also, thermal radiation's in the form of infrared (IR) beams can be received from a living body or a machine in operation and the beamscan be transformed into an image thereof as in a night vision system. As the IR beams are invisible to human eyes, law enforcement groups often use IR scopes in the detection of criminals hiding in the darkness or the people lost in the mountains or in the forests during hiking excursion. These examples represent active and passive applications where the user can and cannot manipulate the beams from the source respectively. The light beams the user receives in the passive mode are generally beyond his control. Manipulation of the energy source for active applications will be discussed more fully below. In the case of light beams utilized in the passive mode, such as in the thermal/IR imaging system, IR energy is emitted by the body or object in accordance with the temperature variations on its surface. Devices, such as night vision systems that utilize thermal imaging technique, rely on temperature gradations on the surface of the source and, as a result, the image formed is usually not clearly defined and is fuzzy. For this reason, a further improvement is required. The quality of the night vision image can be greatly improved by illuminating the target to supplement the thermal radiations by using an expanded IR laser beam. The following is a list of representative potential applications of an expanded laser beam:
1. Remote Sensing and Surveillance of Hazardous Weather Conditions
Doppler radar is currently used to monitor hazardous weather conditions such as hurricanes, tornados, and wind shear. Because the radars utilize narrow-beam electromagnetic waves in detecting the movements of air in the atmosphere, the radar system has to scan the target area in two dimensions, thus causing inaccuracies in the collected data. By expanding a laser beam in one dimension into an extremely wide angle of almost 180 degrees, a large territory can be illuminated by the laser. Rotating or setting the expanded beam into a pitching (up and down) motion greatly increases the area which can be blanketed by the beam. A multiple number of expansion units may be used simultaneously to further increase the intensity of the expanded laser beam to improve detection of objects. Another advantage of the invention is that the laser beam expander and detector system can be installed not only on fixed land installations, but also on mobile units such as airplanes and helicopters, including, but not limited to, remotely controlled unmanned aircrafts such as drones and satellites.
2. Improvement of Night Vision System
An expanded laser beam can be used to improve the thermal/IR imaging system. A laser beam irradiated object produces images which are more clearly defined, as they do not rely solely on the temperature gradation method, and contour lines and shadowed areas of target are more positively defined. When these images are combined with the images formed by the night vision (i.e., thermal imaging) system, which are produced by IR radiated from the target more realistic images can be formed thus improving the quality of images formed by the night vision system alone.
3. Aircraft Identification System
Laser beams spread by the laser expanders that are mounted on top and on bottom of an aircraft and are rotated in accordance with a frequency registered with FAA can easily be detected and identified by other aircrafts in the sky as well as by the air-control stations on the ground. The identification of the aircraft not only ensures the safety of aircrafts in the sky, but also enables FAA to regulate the flight patterns of commercial aircrafts.
4. Collision Avoidance System
Installation of expanded laser beam transmitters on various different vehicles can be used to prevent collision of the vehicles so marked:
a. Aircrafts--Units fitted to top and bottom of an aircraft's fuselage can each be rotated at different frequencies, for example, the top one at frequency A and bottom one at frequency B. When the pilot of a second plane observes frequency A, he recognizes that a plane is flying below him and he can take an immediate evasive action to maneuver his plane upwards; likewise, when he recognizes frequency B, he can move downwards to avoid collision. In the same manner, different transmitters can be attached to the wing tips and/or front and rear of the plane. PA1 b. Marine vessels--Transmitter units installed one on the port side and one on the starboard side of a marine vessel can be used to recognize the presence of that vessel and hazardous objects such as other vessels and icebergs and their directions of travel, depending on which laser signal is received by the vessel, so that the captain of the vessel can take action to avoid collision.
5. Night Vision System for Aircraft Navigation
As mentioned above, night vision systems rely on the temperature gradation method. For this reason, when it is used as a navigation system for aircraft, it can encounter difficulties. For instance, the night vision system cannot detect the terrain configurations of a mountain when the mountain is covered with snow and the atmospheric temperature is almost the same as that of snow (i.e. no temperature gradation). However, expanded laser beams will bounce back from the surface of snow, but not from the atmosphere, so the terrain can clearly be distinguished from the sky, thus presenting an advantageous and improved night vision system for use as a navigation system for aircraft.
6. Tracking of Unlawful Aircrafts
An expanded laser beam can illuminate the entire hemisphere during every 1/2 rotation when directed skyward. For this reason, any unlawful flying objects, such as drug smuggling airplanes that are equipped with no such identification device as described in Item 3, will be illuminated by the laser and detected as soon as they fly into the hemisphere. The flying objects are therefore trackable by the laser beam expander/detector system as long as the system is in operation. This system can therefore be used as an improved aircraft control system to supplement the air-control radars currently in use at airports and ground installations.
7. Homing and Landing Device for Aircrafts
Eye-safe IR laser beams transmitted and rotated at a certain frequency assigned to that location by the air-controller of an airport will allow pilots of airplanes to identify and locate their landing sites even in darkness. This allows the aircrafts to fly back safely to their bases by following the rotating laser signals.
Additional laser beam transmitters can be fixed to the runway area of airport to project the beams skyward, one on port side, one on starboard side and one in between along the center line along which landing should be executed. The units on port and starboard sides would be operated continuously and without rotating or pitching motion, while the one in the middle could be flashed at a certain frequency. The first two units are used to let the pilot of homing aircraft know the boundaries of a landing site, while the flashed beams in the middle would guide the pilot to the point of landing.
8. Rescue Light for Missing Person
Missing persons such as downed pilots or lost hikers can alert the rescue teams as to their location by use of a portable rotating expanded beam unit. The device would project an expanded laser beams covering the entire hemisphere when rotated and it will let the rescue teams, regardless of whether they are in the sky or on land, easily detect the laser signals and locate the exact spot where the downed the missing persons are located.
9. Communication System
If a laser beam is expanded into almost 180 degrees, it can be used as a means of communication by flashing the laser in accordance with codes. Its use is more advantageous over the use of a discrete laser when the message must be delivered to a number of parties simultaneously.
10. LIDAR (Laser Radar)
Conventional LIDAR system utilizes a narrow electromagnetic beam to scan the sky to collect the atmospheric data. For this reason, scanning must be carried out in two directions resulting in a time lapse between the initiation and the conclusion of scanning carried out in a plane, causing some errors in data when a severe atmospheric movement exists. An expanded laser beam system could analyze and provide the data on atmospheric conditions with a faster speed and an improved accuracy, as it is required to scan only in one dimension, thus cutting down the time lapse considerably.
11. Bar Code reader
A conventional bar code reader scans a bar code by utilizing a rotating mirror to deflect a laser beam emitted by a diode laser. For this reason, the scanning speed of the laser beam, that is, the rotational speed of the mirror, should be synchronized with the bar code reading speed. However, as the two speeds are often mismatched in practice, reading of the bar code must often be carried out repeatedly in order to register the correct data. In contrast, a laser beam expanded by the invention can illuminate the entire bar code continuously, thus eliminating the need to synchronize the speed of rotating mirror with the bar code reading speed. Therefore, the new bar code reader utilizing an expanded laser beam can provide correct bar code data at any reading speed and failure in reading the bar code is eliminated.
12. Alignment of Large Structures
A conventional method to align large structures such as ships, buildings, bridges and highways utilizes a single laser beam of small cross-section that is swept in two directions. One dimensional sweep assures the correct alignment of structures on a line, while the sweep in the other direction, usually in vertical direction, is carried out to correct uprightness of the structures. However, it is important that the two sweeps are carried out in at an exact right angle to each other. As the laser beam expanded by the invention generates an arc of almost 180 degrees, it can be used to align the structures in two directions simultaneously. Therefore, it not only cuts the surveying time, but also increases the accuracy in alignment.
Many of the applications of laser beam expanders that employ visible and IR laser sources have been described above. The nocturnal use of IR systems is advantageous, because IR is invisible to human eyes, thus requiring special sensors to detect. For this reason, IR systems have been sought after, developed and improved for applications in the areas such as security and law enforcement where the roles of invisible beams are appreciated. However, the use of expanded laser beams is not limited to visible and IR systems. As visible and IR beams cannot penetrate deep into a body of water, those systems become useless in water-related applications such as the surveillance of ocean and shoreline conditions. However, the visible and IR sources can be replaced with ultraviolet (UV) sources, such as excimer lasers, that are also invisible to human eyes and can be used in nocturnal operations. The following are some applications where expanded UV lasers may be preferred.
13. Surveillance of shoreline conditions
When UV (excimer) lasers are used in the study of ocean conditions, two types of signals are generated by the beams, one from the ocean surface and another from the ocean floor. By analyzing the two signals, surface conditions of ocean as well as depths of ocean floors can be mapped.
14. Detection of Submerged Obstacles
As UV beams penetrate into water, expanded UV laser beams can be used to detect obstacles or submerged objects along shorelines. A standard UV laser, which uses a narrow, circular, high intensity beam to investigate the area of interest requires a two directional scanning which is not only time consuming, but also inaccurate as the plane equipped with the scanning device must fly over the area repeatedly to create an overlapping area scan. Using an expanded laser beam system equipped with a UV laser installed on board an airplane, the UV laser being operated in pitching motion synchronized with the speed of the airplane, only one pass is necessary to cover an area of interest and identify the locations of obstacles or objects under the water below the expanded beam path. Therefore, an expanded beam system would be more accurate and efficient than current conventional methods.
While many applications of expanded laser beams can be identified, suitable systems are not currently available which can produce flattened laser beams of uniform intensity, or laser beams with suitable intensity distributions along widely expanded arcs, which can be utilized in the numerous applications described above. Thus, there is a need for a new optical system which meets the criteria necessary to operate in the above applications. Described herein is a laser beam profile converter that has been invented to serve the purpose.